Photovoltaic solar cells essentially comprise a semiconductor substrate of one conductivity type having a shallow p-n junction formed adjacent to the front surface thereof. The cells require electrical contacts to their front and rear sides to obtain electrical current from the cells when they're exposed to solar radiation. The contact on the front of the solar cell is generally made in the form of a grid comprising a plurality of narrow elongated parallel fingers that extend in one direction and at least one but in most cases two elongate bus bars that intersect the fingers at a right angle. The width, number and spacing of the fingers are arranged so as to expose an optimum area of the front surface of the cell to incident solar relation.
The prior art methods of forming a grid electrode require patterning of the electrode grid on the front surface. One method is to screen print electrodes over a deposited silicon nitride layer, which is used to form the anti-reflection (AR) coating. The screen printed electrode grid, which is typically formed from a silver-filled paste, dissolves the silicon nitride when fired, forming a contact. Such a method forms lines on the order of 100 μm wide that blocks a significant portion of the incident sunlight, and the contact alloys to a depth of a few thousand angstroms. The wide lines and deep contacts are suitable for lower efficiency cells, but not for high efficiency devices that need narrow, shallow contacts. For these devices, patterning usually involves masking, by way of example, photolithography and etching to provide openings in the antireflective coating provided on the front surface of the solar cell so that metalized contacts can be applied directly to the front surface of the solar cell. Such masking adds to the time and cost of producing a solar cell, and is often not economically viable.
There is therefore a need for a contact forming technique for narrow, shallow lines that can be integrated into a mass production manufacturing process in which the contact forming technique does not utilize patterning and thus increases the speed and reduces the complexity with which the contacts are formed.